The brief
The Touran's aircon was blowing warm instead of cold, the front of the engine bay produced a grinding noise whenever the AC compressor engaged, and a check showed the refrigerant pressure was dropping. Three converging signs of a compressor at the end of its life.
The aircon compressor is the pump that drives the whole loop, squeezing the refrigerant so it can shed heat at the condenser. It runs off the engine belt through a clutch. When the clutch bearing wears out it grinds, and a failing compressor sheds metal into the system, which contaminates it. So this isn't a regas job, it's a compressor with a flush.
The diagnosis
The pressure check showed the AC system low on refrigerant. Listening at the compressor confirmed the bearing grinding as the clutch engaged, the internal damage.
That metal contamination is the key thing: a new compressor on its own would be killed by the debris the old one left in the lines, so the loop has to be flushed at the same time. Replacement, not a clean, with a flush.
The work
The remaining refrigerant was recovered, the failed compressor came off, and the AC lines and condenser were flushed to clear the metal debris. A new VAG-spec compressor went on with a fresh receiver-drier.
Then the system was vacuumed to a long, deep pull to draw out every trace of air and moisture, and recharged with the correct weight of refrigerant and the right oil charge.
A check at the vents confirmed cold air at idle, with no grinding when the AC engaged.
The outcome
Cold air at the vents at idle, no grinding when the AC engages, and the system holding its pressure.
The Touran went home with the aircon working the way it should. Doing the flush properly, rather than just bolting on a new compressor, is what stops the second compressor failing the same way as the first, and on a family MPV in Singapore an aircon that actually keeps up is not optional.